I would like read your opinion on rather usual texture: inclusions
of Mg-spinel or hercynite in the core of cordierite porphyroblasts
in quartziferous rocks (generally, gneisses). Sometimes these
porphyroblasts contain also fibrolite needles. The direct contact
between spine/hercynite and quartz is invariably absent.
Spinel-bearing cordierites are known mostly in granulitic gneisses
however without evidences for temperatures above 1000úC. I have
met also hercynite inclusions in cordierite from an assemblage
Qtz+Pl+Bt+Chl+Crd+St+And. The mineral composition and geological
setting (central part of greenstone belt) of this rock indicates
that it did not heat up to 1000úC.
So, spinel or hercynite in silica-saturated rock below this
temperature must be metastable. What can you say about origin of
such inclusions? Why they forms? Why spinel is located in
cordierite, not in other minerals?
Pavel Azimov, PhD
Institute of Precambrian Geology and Geochronology
Russian Academy of Sciences
2, Makarov Embankment
St.Petersburg, 199034, Russia
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